Stainless steels are used in wide fields including automobile members, construction members, and kitchenware as high corrosion resistance materials. As of these applications, wheel cap of automobile, and the like, request a material having both high punch stretchability and high crevice corrosion resistance. Stainless steels are generally grouped, based on the structure of the steel, into four categories: austenitic stainless steels, ferritic stainless steels, austenitic-ferritic stainless steels, and martensitic stainless steels. As of these stainless steels, the austenitic stainless steels represented by SUS304 and SUS301 (specified by Japanese Industrial Standard (JIS)) are most widely used owing to their excellent corrosion resistance and workability. Accordingly, the austenitic stainless steel sheets are generally adopted by the wheel cap of automobile.
Compared with other types of stainless steels, however, the austenitic stainless steels have a drawback of high cost because of large content of expensive Ni, though the steels have high workability.
Furthermore, the austenitic stainless steels likely induce seasoned cracks on working to near the forming limit and have high sensitization to stress corrosion cracking (SCC). As a result, the austenitic stainless steels have a problem in application to portions such as fuel tanks where the requirement for safety is extremely severe. Regarding the martensitic stainless steels, they are inferior in ductility, punch stretchability, and corrosion resistance, though the strength is high, thereby failing to apply them to press-forming.
The austenitic stainless steels represented by SUS301 face a criticism of occurrence of problems, in some cases, such as insufficient corrosion resistance, inducing, in particular, corrosion at gaps between wheel and cap of automobile in coastal zones owing to the salt scattered in wind, and in snow zones owing to the snow-melting salt. In addition, as described above, since seasoned cracks appear on working to near the forming limit, there is a problem of difficulty in application of the austenitic stainless steels to a member having complex shape. Furthermore, the austenitic stainless steels have a problem of high cost because of the Ni content at 6% or more in general grades.
On the other hand, ferritic stainless steels have excellent characteristics. That is, they can increase the corrosion resistance and the crevice corrosion resistance by increasing the Cr content, and they induce very little seasoned cracks and stress corrosion cracking. The ferritic stainless steels, however, have a drawback of inferior workability, particularly inferior balance of strength and ductility, to the austenitic stainless steels. In addition, compared with austenitic stainless steels, the ferritic stainless steels have a problem of very poor punch stretchability and difficulty in forming. The martensitic stainless steels are insufficient in both the punch stretchability and the crevice corrosion resistance.
To this point, there have been proposed technologies for improving the workability of ferritic stainless steels. For example, JP-A-08-020843, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), discloses a Cr steel sheet, or a ferritic stainless steel sheet containing 5 to 60% by weight of Cr, having excellent deep drawability, by decreasing the content of C and N, while adding appropriate amount of Ti and Nb, and a method for manufacturing the Cr steel sheet. Since, however, the steel sheet of JP-A-08-020843 decreases the content of C and N to 0.03% by weight or less and 0.02% by weight or less, respectively to improve the deep drawability, the steel sheet is poor in the strength and is insufficient in the improvement of ductility. That is, the steel sheet has a problem of poor balance of strength and ductility. As a result, when the steel sheet according to JP-A-08-020843 is applied to an automobile member, the necessary sheet thickness to attain the required strength of the member increases, which fails to contribute to weight saving. In addition, the steel sheet has a problem of inapplicability to severe working uses such as punch stretching, deep drawing, and hydraulic forming.
In this regard, the austenitic-ferritic stainless steels which are positioned between the austenitic stainless steels and the ferritic stainless steels have drawn attention in recent years. The austenitic-ferritic stainless steels have excellent corrosion resistance. Owing to the excellent strength and corrosion resistance, the austenitic-ferritic stainless steels are used as the anti-corrosive materials in high-chloride environment such as seawater and in severe corrosive environment such as oil wells. The SUS329 group austenitic-ferritic stainless steels specified by JIS, however, are expensive owing to the content of expensive Ni by 4% or more, by mass (the same is applied in the following), and have a problem of consuming large amount of valuable Ni resource.
Responding to the problem, JP-A-11-071643 discloses an austenitic-ferritic stainless steel sheet having high tensile elongation, by limiting the Ni content to a range above 0.1% and below 1%, and by controlling the austenite stability index (IM index: 551-805(C+N) %-8.52Si %-8.57Mn %-12.51Cr %-36.02Ni %-34.52Cu %-13.96Mo %) to a range from 40 to 115.
There are other trials of decreasing the Ni content in austenitic stainless steels and austenitic-ferritic stainless steels by the addition of large amount of N instead of Ni. An example of these trials is introduced by Yasuyuki Katada, “Manufacture of high N steel by pressurized electro-slag remelting (ESR) process”, Ferrum, vol. 7, p. 848, (2002), describing the method for manufacturing austenitic stainless steel and austenitic-ferritic stainless steel containing substantially no Ni, by the addition of large amount of N.
Alternatively, J. Wang et al. discloses an austenitic-ferritic stainless steel with inexpensive alloying cost, containing substantially no Ni, in “NICKEL-FREE DUPLEX STAINLESS STEELS”, Scripta Materialia, vol. 40, No. 1, pp. 123-129, (1999).
However, the austenitic-ferritic stainless steel sheet disclosed in JP-A-11-071643 does not attain satisfactory ductility, though it does improve the ductility to some extent, and has no satisfactory deep drawability. Consequently, the austenitic-ferritic stainless steel of JP-A-11-071643 has problems of difficulty in application to the uses subjected to an extreme degree of punch stretching and hydraulic forming, and of difficulty also in application to the uses subjected to an extreme degree of deep drawing.
Furthermore, the austenitic-ferritic stainless steel disclosed in JP-A-11-071643 is insufficient in the crevice corrosion resistance because of the large amount of Mn, though it shows high tensile elongation, and the steel has a problem that the punch stretchability is not known. The steel has another problem of poor corrosion resistance at welded part. That is, since the austenitic-ferritic stainless steels are subjected to welding before use depending on their uses, they have to have excellent corrosion resistance at welded part. Since, however, the austenitic-ferritic stainless steel according to JP-A-11-071643 contains 0.1 to 0.3% N which is an austenite-forming element to decrease the Ni amount, the N becomes solid solution at high temperatures at the welded part and surrounding heat-affecting zone, which N solid solution then likely precipitates as a chromium nitride, thereby generating a chromium-depleted zone to deteriorate the corrosion resistance.
According to JP-A-11-071643, furthermore, N is added by the amounts from 0.1 to 0.3% by weight as an austenite-forming element instead of decreasing the Ni content. Consequently, when the cooling rate after the solution annealing is slow, the N precipitates as a chromium nitride to deteriorate the corrosion resistance. The phenomenon is what is called the problem of sensibility, or the deterioration of corrosion resistance owing to the formation of chromium carbide and chromium nitride at grain boundaries, (hereinafter referred to as the sensitization).
Specifically, when finish-annealed sheets having 1.5 mm or larger thickness were air-cooled, the slow cooling rate of the material induced sensitization during the cooling step, thus the corrosion resistance became insufficient in some cases.
Even the materials having less than 1.5 mm in the final sheet thickness raised a problem caused by the sensitization occurred during the annealing of hot-rolled sheet as an intermediate step. That is, the finish-annealed sheets having less than 1.5 mm of thickness are manufactured by, after steel-making and casting, the successive steps of hot rolling, annealing, descaling by pickling, cold rolling, and finish-annealing. In the course of these manufacturing steps, since the material becomes sensible during the air cooling after the annealing of hot-rolled sheet (1.5 to 7 mm in sheet thickness during the annealing), the grain boundaries are preferentially corroded during the succeeding pickling step, and the preferentially-corroded grooves do not vanish even in the cold rolling step, which raises a problem of significantly deteriorating the surface property of the final finish-annealed sheet. To improve the surface property, it is effective to grind the surface after the annealing of hot-rolled sheet using a grinder. The grinding, however, significantly increases the cost.
With the background described above, there is wanted a material that is sensitized very little during cooling step after the solid solution heat treatment.
The means which is disclosed by Yasuyuki Katada, “Manufacture of high N steel by pressurized electro-slag remelting (ESR) process”, Ferrum, vol. 7, p. 848, (2002), contains many cost-increasing causes on operation, even as a simple Ni-decreasing means, such as the necessity of large apparatus for performing pressure melting, and the necessity of electrode for preliminarily melting material. Furthermore, the means has to attain both the punch stretchability and the crevice corrosion resistance even when simply the Ni is replaced by N.
Also for a means disclosed by J. Wang et al. in “NICKEL-FREE DUPLEX STAINLESS STEELS”, Scripta Materialia, vol. 40, No. 1, pp. 123-129, (1999), since the simultaneous addition of large amount of Mn (as large as 10% by mass) and N (0.35 to 0.45% by mass) to decrease the amount of Ni is done, the hot workability is not sufficient and the cracks and flaws likely occur during hot working. The disclosed means has many cost-increasing causes such as necessity of surface grinding and of steel cut-off, through the alloy cost is low.
An object of the present invention is to provide an austenitic-ferritic stainless steel which has high formability with excellent ductility and deep drawability.
Another object of the present invention is to solve the above-described problems in the related art, and to provide a austenitic-ferritic stainless steel which has both the high punch stretchability and the high crevice corrosion resistance while decreasing the amount of Ni.
A further object of the present invention is to solve the above-described problems in the related art, and to provide a austenitic-ferritic stainless steel which has excellent corrosion resistance at welded part at a relatively low cost while saving the Ni resources.
A still another object of the present invention is to solve the above-described problems, and to provide an austenitic-ferritic stainless steel sheet which has excellent intergranular corrosion resistance.